1. Field of the Invention
The present invention relates to a semiconductor device and to a method of manufacturing the same and, more particularly, to a semiconductor device having a nonvolatile semiconductor memory (FeRAM: Ferroelectric Random Access Memory) using ferroelectric material as a dielectric film of a capacitor and a nonvolatile semiconductor memory (DRAM: Dynamic Random Access Memory) using high-dielectric material as the dielectric film of the capacitor, and to a method of manufacturing the same.
2. Description of the Prior Art
In the FeRAM, the multi-layered wiring technology used in other electronic devices is being employed to meet the demand for the higher integration of the device. However, since the ferroelectric material used in the FeRAM is exposed to the reducing atmosphere containing the hydrogen when the interlayer insulating film, the tungsten plug, the cover film, etc. are formed, the ferroelectric material is easily damaged by the formation of the multi-layered wiring structure.
In order to suppress the degradation of the ferroelectric film constituting the capacitor due to the reducing reaction, several trials were carried out.
For example, in FIG. 1 of Patent Application Publication (KOKAI) Hei 7-111318, it is set forth that, the protection film made of aluminum nitride which is formed above the upper electrode of the capacitor is possible to prevent the reduction of the ferroelectric film of the capacitor caused when the reducing gas permeates through the upper electrode. Also, in FIG. 8 of the same Publication, it is set forth that the protection film is formed on the wiring connected to the upper electrode of the capacitor and on the insulating film covering the capacitor. But no recitation about constituent material and the particular operation of the insulating film is given.
Also, in Patent Application Publication (KOKAI) Hei 9-97883, it is set forth that the lower electrode and the dielectric film constituting the capacitor are formed, then the dielectric film is covered with the insulating film, then the opening for exposing the dielectric film is formed in the insulating film, then the upper electrode of the capacitor is formed in the opening and on the insulating film, and then the protection film having the double-layered structure consisting of titanium and titanium nitride is formed on the upper electrode. This protection film has a function for preventing the diffusion of the hydrogen into the capacitor and the permeation of the moisture into the capacitor.
In FIG. 1 of Patent Application Publication (KOKAI) Hei 7-235639, it is set forth that the lower electrode and the dielectric film constituting the capacitor are formed, then the capacitor is covered with the insulating film, then the opening for exposing the upper electrode is formed in the insulating film, and then the wiring having the double-layered structure containing the titanium tungsten film is formed in the opening and on the insulating film. Also, in FIG. 2 of the same Publication, it is set forth that the water resisting layer made of silicon nitride is formed on the titan tungsten film over the capacitor except an area of the upper electrode of the capacitor. This water-resistant layer is formed to shut off the permeation of the moisture from the region on which the wiring is not formed.
Also, it is set forth on 17-th Ferroelectric Material Application Conference, Preprint, pp. 17-18 that the metal wiring connected to the capacitor is formed and then the alumina (Al2O3) film for covering the metal wiring is formed over the overall area of the substrate.
By the way, the structure in which the first level wiring is connected to the upper electrode of the capacitor is disclosed in above references, but it is not set forth to form further second and third level wirings above the capacitor.
Accordingly, since the capacitor is exposed further to the reducing atmosphere during the step of forming the multi-layered wiring above the capacitor, there is the possibility that the degradation of characteristics of the capacitor cannot be satisfactorily suppressed by the protection structure of the above capacitor in above references.
The degradation of the imprint characteristic becomes the greatest problem out of degradations of the ferroelectric capacitor due to the reducing atmosphere. The degradation of the imprint characteristic is such a problem that, if one signal (e.g., xe2x80x9c1xe2x80x9d) is written into the ferroelectric capacitor, then the ferroelectric capacitor is left for a certain time as it is, and then an opposite signal (e.g., xe2x80x9c0xe2x80x9d) is written into the capacitor, the opposite signal cannot be read out. In other words, the degradation of the imprint characteristic signifies such a situation that, since the signal in the one direction is imprinted into the capacitor, it is difficult to write the opposite signal into the capacitor.
In the 2-transisitors/2-capacitors type FeRAM, after the positive signal is written into one of two pair of ferroelectric capacitors and the negative signal is written into the other, a difference of the polarization charge between the two capacitors is set to Q.
Then, a difference of the polarization charge between the two ferroelectric capacitors obtained after the ferroelectric capacitors are baked at 150xc2x0 C. for 88 hours is defined as Q(88) xcexcC/cm2, and a degradation rate of the difference Q between the capacitors obtained after an e time (e=natural logarithm) lapsed is defined as a xe2x80x9cQ ratexe2x80x9d, both are used as indices of the imprint characteristic. In other words, it is understood that, as a value of Q(88) is increased larger and an absolute value of the Q rate is reduced smaller, the imprint characteristic becomes excellent much more.
Now the reason to evaluate the ferroelectric capacitor at 150xc2x0 C. for 88 hours is to assure the 10-year use of the FeRAM under the circumstance of 55xc2x0 C. Details are set forth in S. D. TRAYNOR, T. D. HADNAGY, and L. KAMMERDINER, Integrated Ferroelectrics, 1997, Vol.16, pp.63-76.
When the degradation of the characteristics of the capacitor due to difference in the wiring structure on the ferroelectric capacitor are evaluated based on the evaluation of the degradation of such ferroelectric capacitor, results shown in Table 1 were obtained by the present inventor""s experiments.
In Table 1, the state that the first metal wiring is connected to the upper electrode of the ferroelectric capacitor is shown as xe2x80x9cafter formation of the ferroelectric capacitorxe2x80x9d. Also, the state that the second metal wiring is formed on the ferroelectric capacitor is shown as xe2x80x9cafter formation of the second layer metal wiringxe2x80x9d. In addition, the state that the third layer metal wiring and the cover film are formed on the ferroelectric capacitor is shown as xe2x80x9cafter formation of the third layer metal wiring+the cover filmxe2x80x9d. The measurement of Q was carried out under the condition applying the voltage of 5 V to the ferroelectric capacitors.
According to Table 1, the Q rate is not so increased until the second layer metal is formed, and thus the degradation of the imprint characteristic is small. However, after the third layer metal wiring and the cover film are formed, the Q rate is increased and thus the degradation of the imprint characteristic appears.
The main causes of the degradation of the imprint characteristic are the CVD process applied to form the tungsten in the reducing atmosphere and the CVD process applied to form the cover film made of silicon nitride in the reduced atmosphere.
Accordingly, with the increase in the number of the wiring layer of the multi-layered wiring structure, the degradation of the imprint characteristic is also increased and thus the degradation of the capacitor characteristic is caused.
It is an object of the present invention to provide a semiconductor device having a structure that is capable of suppressing degradation of a ferroelectric or high-dielectric capacitor formed under a multi-layered structure, and a method of manufacturing the same.
According to the present invention, the semiconductor device has the first protection film which is covering the surface of the ferroelectric or high-dielectric capacitor, the second protection film formed to cover the capacitor through the first wiring formed above the capacitor, the second wiring formed over the second protection film, the third protection film formed to cover the capacitor over the second wiring, and the third protection film is set to the earth potential.
According to this, even when the insulating films and the conductive films are formed or etched over the ferroelectric or high-dielectric capacitors in the reducing atmosphere, the ferroelectric or high-dielectric films of the capacitors can be protected from the reducing atmosphere by the first protection film, the second protection film, and the third protection film underlying the films which are subjected to the forming or etching processes.
The second protection film or the third protection film can prevent the reducing gas over the second or third protection film from permeating in the ferroelectric or high-dielectric capacitor, but the second or third protection film cannot prevent the moisture or the hydrogen existing under the second or third protection film from entering into the capacitor.
Therefore, in order to prevent the reduction of the ferroelectric or high-dielectric capacitors, either a combination of the first protection film and the second protection film or a combination of the first protection film and the third protection film is indispensable. When all the first protection film, the second protection film, and the third protection film are provided, the reduction of the ferroelectric or high-dielectric capacitors can be excellently prevented. Then, these protection films improve the imprint characteristic of the ferroelectric capacitor satisfactorily and also improve the retention performance peculiar to the FeRAM.
Also, as the third protection film is set to the earth potential, the mutual induction between the second wirings under the third protection film, e.g., the bit lines, under the third protection film is prevented and also the fluctuation of the electlic potential of the second wirings can be suppressed, whereby the performance of FeRAM or DRAM can be improved. In addition, the third protection film that is set to the earth potential prevents the hydrogen ion over the third protection film from permeating into the ferroelectric capacitors. The hydrogen ion is generated when one film is growing over the third protection film.
In case the first protection film and the second protection film are formed of alumina, the imprint characteristic of the ferroelectric capacitors can be improved if the thickness is set to 15 to 100 nm or the helicon sputter method is employed.